The present invention relates to substrates for use in semiconductor packages, and more particularly, to a substrate for accommodating chips and passive components in a semiconductor package.
With improvement in functionality and processing speed of electronic products, it increasingly desires to integrate a semiconductor package with passive components such as capacitor, resistor or inductor, so as to raise or stabilize electricity and functions of the electronic products. However, in response to profile miniaturization of the electronic products, area on a substrate for accommodating the passive components is accordingly getting reduced in the semiconductor package. Therefore, how to desirably incorporate sufficient passive components on the limited area of the substrate but not to adversely affect the trace routability of the substrate, is a critical problem to solve in the semiconductor industry.
Normally, solder paste is applied on solder pads on a substrate, for allowing a passive component to be bonded and electrically connected to the solder pads by the solder paste in proceeding of a reflow soldering process. During reflow soldering, thermally-induced variation in surface tension makes the solder paste in the solder pads swell up to form a bulged surface; this undesirably causes shifting or floating effect on the passive component, which effect is severe especially for those decreasingly dimensioned passive components. Once the passive component is shifted in position, it easily comes into contact with other nearby elements or bonding wires in a restricted space; this therefore results in the short circuit problem. Another problem often occurring during reflow is tombstone effect of the passive component. Once the passive component is positionally erected as dragged by the solder paste, it cannot be properly electrically connected to the solder pads on the substrate, thereby leading to degradation in electricity quality of fabricated products.
Accordingly, U.S. Pat. No. 5,311,405 entitled xe2x80x9cMethod and Apparatus for Aligning and Attaching a Surface Mount Componentxe2x80x9d renders solutions for the foregoing shifting and tombstone problems. In this prior art, a passive component has its two ends respectively attached to two opposing solder pads, and each of the solder pads is composed of an elliptic portion and a conical portion connected to the elliptic portion, with the conical portions of the two solder pads facing each other. Solder paste is applied onto the elliptic portions of the solder pads for attaching the two ends of the passive component thereto. In a reflow soldering process, the solder paste flows from the elliptic portions to the conical portions of the solder pads; this generates a dragging force for recovering the passive component back to the right position, and makes a longitudinal axis of the passive component aligned with an imaginary line that passes through centers of the two solder pads, so that the passive component can be free of position shifting concern during reflow.
However, there are several drawbacks for the foregoing disclosure of U.S. Pat. No. 5,311,405. First, two solder pads are necessarily formed in an elliptic shape for disposing the two ends of the passive component thereon, thereby making the area of the substrate more occupied by the two solder pads than the use of conventional solder pads; this undesirably affects the trace routability of the substrate, and is definitely not advantageous for profile miniaturization of the substrate. Moreover, since the conical portions of the two solder pads are closely spaced to each other, short circuit easily occurs due to the solder paste leaking out of the conical portions and making the solder pads be in electrical contact with each other. One solution is to increase the distance between the two solder pads; however, this results in more area on the substrate being occupied by the solder pads, and makes trace routing on the substrate more difficult to implement, as well as even needs to undesirably increase the size of the substrate. Furthermore, since the passive component is attached to the solder pads by using the solder paste, if the amount of solder paste is not evenly applied to the solder pads respectively, the passive component may suffer uneven dragging force from the solder paste during reflow, and then is easily subjected to tombstone effect. In addition, shape and structure of the solder pads used in the foregoing patent cannot be achieved simply by utilizing conventional equipment and processes, and a conventional stencil for applying the solder paste is also not suitably adopted; therefore, costs and process complexity in fabrication would be undesirably increased.
A primary objective of the present invention is to provide a substrate for accommodating a passive component, which can effectively prevent the occurrence of position shifting or tombstone effect on the passive component during bonding the passive component onto the substrate.
Another objective of the invention is to provide a substrate for accommodating a passive component, which can effectively prevent the occurrence of position shifting or tombstone effect on the passive component, without affecting the trace routability of the substrate or increasing the usage area on the substrate.
A further objective of the invention is to provide a substrate for accommodating a passive component, which can effectively prevent the occurrence of position shifting or tombstone effect on the passive component, and use currently available equipment and processes for bonding the passive component onto the substrate.
In accordance with the above and other objectives, the present invention proposes a substrate for accommodating a passive component, comprising: a core layer having a surface being formed with a chip attach area for mounting a chip thereon, and a trace forming area surrounding the chip attach area for disposing a plurality of conductive traces on the trace forming area; a solder mask layer applied onto the trace forming area, for hermetically encapsulating the conductive traces; and at least a pair of solder pads formed on the trace forming area and partly exposed to outside of the solder mask layer, each of the solder pads being formed with a central recess at least at a central position thereof, allowing the core layer to be partly exposed through the central recesses of the solder pads. During a reflow soldering process for bonding a passive component onto the solder pads by means of solder paste, the solder paste comes into contact with the exposed part of the core layer through the central recesses of the solder pads. Since the solder paste is not wettable with the material for making the core layer, it forms a recessed top surface due to surface tension of the solder paste, and thereby generates a downward and convergent dragging force for concentrating the solder paste at central positions of the solder pads, which dragging force can thus securely position the passive component above the solder pads without producing shifting or tombstone effect.
Besides the central recess, each of the solder pads can also be formed with at least a pair of rib-like recesses that are symmetrically positioned in association with the central recess and extend toward a periphery of the solder pad; which rib-like recesses can further help enhance the downward and convergent dragging force generated by the movement of the solder paste. The central recess is not particularly limited in shape; circular or rectangular shape is also suitably applicable. And, the central recess is not particularly dimensioned, with the only requisite of not interfering with electrical connection between the passive component and the solder pads. In addition, the central recess is dimensionally equal to or larger than the rib-like recesses in diameter, depending on practical requirements.